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Quantum Materials in Extreme Conditions

Kolloquium der Abteilung 2

A chapter in metrology history was written in 2019 when the piece of Pt-Ir alloy known as Le Grand K was retired, marking the end of an era of measurement standards as tangible objects that predated the French Revolution. This happened thanks to the combined phenomenal progress in the understanding of quantum phenomena in materials and the development of novel quantum nanotechnologies. A good example is the quantum metrology triangle consisting of the quantum Hall effect (QHE) in 2D electron gases, the Josephson effect in SNS multilayer structures, and the single-electron pump in semiconducting quantum dots (QDs) that provide direct access to the elemental electronic charge (e) and the Planck constant (h) for the definition of the base units kilogram and Ampere respectively. The Quantum Materials behind these realizations are, however, just the most recent champions in an unrelenting search among systems where underlying correlations between charge, spin, and lattice, determined by nature, geometry, topology, and dimensionality are very strong. A better understanding of their physics often requires experimentation in extreme conditions of pressures, temperature, and magnetic fields, used to tune the quantum mechanical phenomena, as well as to simplify the phase-space landscape with a single dominant energy scale. In this seminar I will discuss recent research on Dirac materials carried out in extreme pulsed magnetic fields to 60T, with emphasis in the Weyl semimetal system BaMnSb2, and the quantum spin liquid candidate a-RuCl3, a system predicted to host the elusive charge-neutral Majorana Fermion. The former system is a realization of the QHE in the bulk (3D) that opens a new window to the determination of  h/e2, the later a demonstration of entropy fractionalization and a thermal Hall effect that appears as a candidate pathway to obtain the ratio between h and the Boltzman constant k in the solid-state.